The invention relates generally to HVAC (heating, ventilating and air conditioning) systems and more particularly to systems and methods for controlling temperature and/or air flow in a dual duct system.
The temperature and ventilation of an area within a building may be controlled through the use of a dual duct terminal box. The terminal box typically includes a hot air inlet duct, a cold air inlet duct, a mixing area where mixing of the hot and cold air occurs, and an outlet duct for passing the mixed air to the area. The temperature and ventilation for the room may be controlled by modulating the air flow rate of warm or cool air supplied to the mixing area. This is typically accomplished by the use of a damper or valves in each of the hot air inlet duct and the cold air inlet duct which are typically controlled by a control system. The dampers are used to regulate the rate of air flow exiting the mixing box and the air temperature exiting the mixing box. Each damper may be positioned in a separate air duct.
Several systems are known for controlling the dampers to obtain a desired comfort level within the room. One known system involves treating the HVAC system as two separate single-input single-output (SISO) systems wherein one control loop operates one damper, usually the cold air damper, to regulate the total air flow while another control loop operates the other damper, such as the hot air damper, to control the temperature in the room. However, a problem arises with such a system since increasing the air flow using the cold air damper will also reduce the temperature of the air. The control system for the temperature then determines that the air temperature is too low and, consequently, opens the hot air damper which increases the total flow and leads to the cold air damper closing again. As a result, the control performance of the system tends to be poor since the system does not hold temperature and flow set points very well.
Another problem arises when one damper reaches an end of its stroke (i.e., in a fully open or fully closed position). At such a point, the HVAC system loses control of the variable associated with the damper. For example, if the damper is the air flow control damper, the control loop for operating that damper reaches a maximum condition so that the damper position can not be changed to properly effectuate the necessary air flow requirement.
Another known approach for controlling dual duct systems is to mechanically link the hot and cold dampers to control air temperature and to add a separate flow control damper in the outlet duct to control air flow to the area. However, the added complexity of the mechanical linkage between the hot and cold dampers typically reduces system reliability by increasing the number of moving parts. Also, the additional flow control damper increases the cost and control complexity of the control system.